Now we can understand why the Gemara could not derive this from Yosef. Yosef lived before we received the Torah on Har Sinai. Although the avos and their offspring kept the Torah, it was not through any oath; their Torah observance was without any oath to do so. Thus, had anyone taken an oath to perform an action that was a mitzvah, the oath would have surely been valid. We would not be able to deduce from this that if one who lives after the Torah was given on Har Sinai takes an oath, the oath would be valid – in view of the fact that we swore to keep the Torah.